Thursday, February 09, 2006

The bear-eating pythons of Borneo

“After being poked with a stick, it fled into a nearby stream, producing, as it twisted, the sound of breaking bones” (Fredriksson 2005, p. 166). While I don’t want people to think that I’m strangely obsessed with the macabre details of unusual predator-prey relationships (viz, see the giant killer eagle post), I will admit that it’s lines like that that prove especially rewarding when reading behavioural studies on predation. This excerpt is from Gabriella Fredriksson’s remarkable recent paper on the predation of Sun bears Helarctos malayanus by big Reticulated pythons Python reticulatus. Yet again, I am amazed by the habits of a big tetrapod predator.. I mean: pythons killing bears?

Granted, Helarctos is the smallest living bear, with Nowak giving total length as c. 1-1.5 m, shoulder height as 70 cm, and weight as 27-65 kg. Though it’s been tradition in recent decades to sink all bears into Ursus, Helarctos is used here as ‘the monophyly of Ursus is not assured, which supports continuing calls for a revision of the genus-level taxonomy of ursids’ (Bininda-Emonds et al. 1999).

Fredriksson is a Netherlands-based Helarctos specialist who has previously published reports on their conservation and on their interactions with humans, and as part of her research she has been looking after confiscated cubs, later releasing them into suitable habitat. These bears wear radiocollars. Two attacks by retics on Helarctos are reported in the paper. One is an assumed attack: a 31-kg bear was sleeping in a tree, called out in distress during the night (Helarctos is diurnal), and was later observed with wounds suggestive of a snake attack. Other possible attackers were eliminated from consideration, though the presumed snake itself was not observed.

The second case is, well, just a little more convincing. In July 1999 a second radiocollared bear (weight 23 kg) remained motionless for about 4 hrs, a period of time suggestive either of a dropped radiocollar, or of death. Fredriksson tracked the collar and found… “that the signal was being emitted from the stomach of a large python”. Because the collar still functioned, Fredriksson and her colleagues and helpers could now track the snake, though to be honest it didn’t do much over the next few days. They ended up catching it and holding it in captivity in the hope that the collar would be regurgitated. The snake only escaped once (and had to be re-tracked). By late October the snake still had the collar inside it, so the decision was made to remove it surgically. All of this went to plan and the snake was fine: it was released in late November. I was relieved that at no point in the paper was the term “dispatched” or “euthanized” used… granted, all field biologists will tell you that it is not their business to interfere with acts of predation carried out upon their study subjects, but, in practice, things very occasionally work out differently.

At a very impressive 6.95 m and 59 kg, this snake was about three times bigger than the bear it ate, and thus quite conceivably capable of what it did. Furthermore, the bear was reportedly in poor condition, being underweight due to a local fruit shortage and suffering stress from having nursed a small cub (and, sad to say, the cub was no-where to be found after the adult bear’s death). Having said all that, killing and eating a bear – big and sharp curved claws, strong jaws and all that – still isn’t going to be a picnic. It probably doesn’t happen often, though maybe it happens more often than we would have thought. Fredriksson cites one other reported instance, also on Borneo.

Though this is the first technical report on bear predation by pythons, other macro-mammal predation acts have been long known. Isemonger (1962) reported cases where African pythons killed and swallowed antelopes that must have weighed around 20 kg, a 32-yr-old man was allegedly eaten by a 7-m retic in 1998 (Auliya unpublished, cited in Fredriksson 2005), and Shine et al. (1998) reported a case of a retic swallowing a pig (of unstated species) that weighed 60 kg. If the latter case is valid (it was only reported to Shine et al. as a pers. comm. from a Sumatran snake-skinner, and is thus an unverified anecdote), then it would be a new world record, as the largest verified mammal meal according to Carwardine’s Guinness Book of Animals Records was a 59 kg impala eaten by an African rock python Python sebae.

But it seems that some big pythons are sometimes over-ambitious, attempting to swallow animals that are too big even for their distensible jaws. So in the 1995 Malaysian case of a 6.65 m python that tried to swallow 29-yr-old Ee Heng Chun, the snake failed to complete the swallowing (a photo depicting this attempted predation is shown above). This snake reportedly weighed 140 kg, and after being scared away from the body was shot dead by police. Other snakes may consume animals far too large for their own digestive abilities, then becoming immobilised and prone to heatstroke (Klauber 1982), or may swallow another snake powerful enough to administer a dangerous bite to the swallower’s stomach wall.

Other pythons have made the mistake of swallowing horned antelopes, with the result being that the antelope’s horns have fatally pierced the stomach and body wall (Mattison 1995). Such piercings are not always fatal: remarkably, the injuries may heal after the offending horns drop off as the prey’s body decomposes inside the snake (Isemonger 1962). ‘Death by piercing’ isn’t limited to big snakes – Klauber (1982) reported that horned toads Phrynosoma may kill snakes that try to swallow them (the lizard’s horns perforate the snake’s throat) and Ramírez-Bautista and Uribe (1992) described the case of a Lyre snake Trimorphodon biscutatus (a terrestrial colubrid) that died after the spiny tail scales of a Spiny-tailed iguana Ctenosaura pectinata pierced the snake’s stomach and oesophagus.

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About Me

Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. YOU ARE CURRENTLY AT A VERY OLD VERSION OF TET ZOO: for modern articles, go to: http://blogs.scientificamerican.com/tetrapod-zoology/ And check out the Tet Zoo podcast at tetzoo.com!